1. Field of the Invention
The present invention relates to a method for representing logic changes of state of a plurality of adjacent circuit nodes in an integrated circuit, and in particular to such a method whereby such changes of state can be represented in a logic image by the use of a pulsed electron probe of a scanning electron microscope.
2. Description of the Prior Art
Circuit analysis and testing during the development phase of microprocessors and microcomputers with increasingly high data capacities requires testing methods and apparatus which can accommodate the increasingly complex and crowded conductor tracks which are present on the integrated circuits employed in such devices. In particular, conventional test technology cannot reliably produce an image for representing the changing logical status of closely adjacent tracks.
The concept of examination and testing of complex integrated circuits by the use of a scanning electron microscope is known to those skilled in the relevant technology. The concept of stroboscopic voltage coding is described, for example, in the article "Beitr/a/ ge zu Fortschritten in der Elektronenstrahlmesstechnik," Feuerbaum, Beitr/a/ ge elektronenmikroskopischer Direktabbildung Oberfl/a/ chen 11, pages 67-71, 1978 and the concept of logic state mapping is described, for example, in the articles "Electron Beam Testing of Microprocessors" Chrichton et al, Digest of Papers, 1980 IEEE Test Conference, pages 444-449, 1980 and "Elektronenstrahl pr/u/ ft elektrische Potentiale in integrierten Schaltungen," Fazekas, Technisches Messen, 48 1981 1, pages 29-35. These known techniques permit a simultaneous imaging of logical status changes at longer tracks which proceed parallel to one another. The methods and techniques described in these articles cannot, however, be applied for checking of short track sections. Logic status changes at such short track sections must be individually measured track-by-track either with mechanical test probes or with the electron probe. A simultaneous measurement of the status changes at short tracks is, however, difficult if not impossible to achieve with mechanical test probes.